Vibrational CARS measurements in a near-atmospheric pressure plasma jet in nitrogen: II. Analysis

TL;DR

This study analyzes vibrational dynamics in a nitrogen plasma jet using CARS measurements, comparing experimental results with theoretical models and simulations to understand vibrational excitation and relaxation processes.

Contribution

It provides a detailed comparison of experimental vibrational excitation data with theoretical rate coefficients and kinetic simulations in a near-atmospheric pressure plasma.

Findings

Vibrational excitation matches predictions from resonant electron collision rates.

Vibrational populations evolve over microsecond to hundred-microsecond timescales.

Electronically excited states have negligible influence on vibrational populations.

Abstract

The understanding of the ro-vibrational dynamics in molecular (near)-atmospheric pressure plasmas is essential to investigate the influence of vibrational excited molecules on the discharge properties. In a companion paper, results of ro-vibrational coherent anti-Stokes Raman scattering (CARS) measurements for a nanosecond pulsed plasma jet consisting of two conducting molybdenum electrodes with a gap of 1 mm in nitrogen at 200 mbar are presented. Here, those results are discussed and compared to theoretical predictions based on rate coefficients for the relevant processes found in the literature. It is found, that during the discharge the measured vibrational excitation agrees well with predictions obtained from the rates for resonant electron collisions calculated by Laporta et al [PlasmaSources Science and Technology 23 065002 (2014)]. The predictions are based on the electric field during the discharge, measured by EFISH and the electron density which is deduced from the field and mobility data calculated with Bolsig+. In the afterglow a simple kinetic simulation for the vibrational subsystem of nitrogen is performed and it is found, that the populations of vibrational excited states develop according to vibrational-vibrational transfer on timescales of a few $\mu$s, while the development on timescales of some hundred $\mu$s is determined by the losses at the walls. No significant influence of electronically excited states on the populations of the vibrational states visible in the CARS measurements ($v \lesssim 7$) was observed.